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Presuming the range information would be stored in the root segment, memory bounding may not be necessary. This data is rewritten frequently in normal operation, such as for a refresh or the addition of an alternate. If we are willing to do a rewrite for a range request then we can have much weaker space bounds on the memory consumed by the range tracking. This would require a read from disk on a cache hit but that is commonly done now. The key difference would be treating a cache "miss" differently, although the mechanism used for an alternate miss may work as well.

Plugin Implementation

An alternative to modifying the cache is to implement a partial object caching plugin on top of the existing cache. The fundamental idea here is that the plugin fetches fixed-size "chunks" from the origin and uses those to compose the range requested by the client. For each client request, the plugin

• determines the set of chunks that overlap the range
• issues a new set of origin requests (in parallel) to fetch the corresponding chunks
• rewrites the cache key for each chunk so they are all stored independently
• creates a transformed client response by composing the correct byte ranges from the chunks

This would be a fairly complex plugin, but it has the advantage of not changing the cache format and minimizes the risk of breaking other features.